Benzene, ketones, esters, alcohols, ethers and halogenated hydrocarbons and other organic compounds in the chemical industry, pesticides, pharmaceutical and other industries are widely used as organic solvents. Due to their low boiling point and high volatility, large amounts of organic waste gas are routinely produced in their production and application processes. Direct evaporation or gasification of Volatile organic compounds (VOCs) not only results in the waste of resources, but also lead to a series of problems in health, environment, and safety. One mainstream technologies for industrial organic waste gas treatment is fixed bed adsorption; however, because adsorption is an exothermic process, the adsorption exotherm causes a temperature increase of the fixed bed, which adversely affects the adsorption, reduces the adsorption capacity, and even creates safety risks. As reported in literature, when activated carbon was used to adsorb acetone at a concentration of 50 mg/L, the maximum temperature during the process reached 40° C.; when the acetone concentration is 100 mg/L, the maximum temperature rose to 80° C. (P. Le Cloirec, P. Pré, F. Delage, S. Giraudet, Visualization of the exothermal VOC adsorption in a fixed-bed activated carbon adsorber, Environmental Technology, 2012, 2 (33):285-290). High-temperature increases not only reduce the adsorption capacity of activated carbon in acetone, but also lead to serious deterioration of activated carbon, and even fire and other safety incidents.
Faced with the challenge to neutralize the negative impact of the exothermic process during adsorption, solutions in prior arts are concentrated on modifying the internal structure or the container wall heat-exchange structure of the fixed bed, in order to accelerate the outward heat transfer thus the lower temperature of the bed. For example, heat transfer pipes can be included within the bed that can facilitate heat exchange with outside environment; cold fluid moves through the pipes and removes heat through cross-wall heat transfer and maintain the bed temperature approximately constant. For example, Chinese patent application number 201010175816.X, published on Nov. 16, 2011, discloses an adsorption bed and an organic waste gas purification system associated with the adsorption bed. The adsorption bed includes a front wind cover, a bed body, and a rear wind cover. The front wind cover is provided with an adsorption inlet and a desorption outlet; the rear wind cover is provided with an adsorption outlet and a desorption inlet; the bed body comprises a plurality of activated carbon layers. The front windshield is provided with an air collecting manifold, and a plurality of horizontally extended collecting slots, each of the collecting slots is opposite to an activated carbon layer, all the collecting slots are connected to the gas collecting manifold, and the air collecting manifold is connected to the desorption outlet. The invention also provides an organic waste gas purification system using the adsorption bed. The adsorption bed and the organic waste gas purification system of the invention can effectively avoid the excessive concentration of heat in the adsorption bed during the desorption process; the excessive concentration might have resulted in accidents such as spontaneous combustion of gas or activated carbon fire. However, because of commonly used inorganic and organic adsorbents, such as activated carbon, are poor heat conductors with inefficient heat transfer capabilities, the method disclosed in this patent requires large numbers of pipes to increase heat-exchanging area and heat-exchanging capacity in order to effectively lower bed temperature. As a result, the effective filling space for adsorbent within the adsorption bed is reduced, equipment investment cost is increased, and treatment capacity is reduced.
The fixed bed process is the prevalent process in organic waste adsorption treatment. Because the adsorption of VOCs on the adsorbent is an exothermic process, especially when dealing with high concentrations VOCs, the release of heat often causes a temperature increase of the fixed bed. The temperature increase not only adversely affects the adsorption and reduces adsorption capacity, but also may bring safety hazard and cause a fire, explosion or other accident. Therefore, reducing the temperature of the bed effectively has always been a key technical issue to be solved in this field.